[... The Rhesos], although it places Rhesos' arrival, as the Iliad does, during a night when Achilles has withdrawn and the Trojans have the upper hand, seems to pose a question as to why an ally would arrive so late in the war. When the messenger has informed Hektor that Rhesos and his army have arrived, Hektor responds that because the Trojans are now winning they will find many more allies willing to help, but he does not want their help since they were not there when the Trojans were struggling (Rhesos 319-326). His words put a political spin on what in the epic tradition is a narrative pattern. When Rhesos arrives, Hektor immedately rebukes him for arriving so late and refuses several potential excuses (396-421). Rhesos admits that he is late in coming and gives a long explanation of factors that have detained him (422-443). This agōn scene between Hektor and Rhesos thus makes the traditional narrative of the late-arriving ally an object of considerable scrutiny in the play.
–Casey Dué & Mary Ebbott, Iliad 10 and the Poetics of Ambush (2010)